1. Field of the Invention
The present invention is directed to particular aqueous phenol formaldehyde resoles that are produced by the base catalyzed condensation of phenol and formaldehyde. These phenolic resoles are especially useful in preparing phenolic foams having both a low k value and excellent fire properties, compressive strength, density, friability and other properties necessary for insulation board. The invention is also directed to foamable phenolic resole compositions prepared using the aqueous phenolic resoles and phenolic foams made from the compositions.
2. Prior Art
Phenolic foams prepared from phenol formaldehyde resoles have been known for many years. It is generally agreed that phenolic foams have the best fire rating of any known foam insulation. Phenolic foams do not burn even when contacted by the flame of a blow torch and give off minimal amounts of toxic gases. Phenolic foams can stand temperatures of 375.degree. F. without serious degradation. Phenolic foams have an ASTM E-84 Steiner Tunnel Flame Spread Rating of about 5, a Fuel Contribution of about 0 and a Smoke Rating of about 5.
Despite these advantages and generally favorable econonics, phenolic foams have not heretofore penetrated the thermal insulation market. One of the main reasons phenolic foams have not been successful is that phenolic foams made heretofore have exhibited an unsatisfactory initial thermal conductivity or an undesirable increase in thermal conductivity over time. Additionally, the compressive strength of prior art phenolic foams is not as high as desirable for normal handling. It has also been reported that prior art phenolic foams have serious problems with friability and punking.
The general composition and method for preparing phenolic foam are well known. Generally, a foamable phenolic resole composition is prepared by admixing an aqueous phenol formaldehyde resole, a blowing agent, a surfactant, optional additives and an acid curing agent into a substantially uniform mixture. The curing catalyst is added in amounts sufficient to initiate the curing reaction which is highly exothermic. The exotherm of the curing reaction vaporizes and expands the blowing agent thereby foaming the composition. The foaming process is preferably performed in a closed mold.
The general method for the continuous manufacture of phenolic foam insulation board is as follows. The foamable phenolic resole composition is prepared by continuously feeding into a suitable mixing device the aqueous phenol formaldehyde resole, the blowing agent, the surfactant, the optional additives, and the acid curing catalyst. The ratio of these ingredients is varied depending on the density, thickness, etc. desired in the final product. The mixing device combines these ingredients into a substantially uniform mixture which is continuously applied evenly onto a moving substrate, usually a protective covering such as cardboard which adheres to the foam. The foaming composition is usually covered with another protective covering such as cardboard which becomes adhered to the phenolic foam. The covered foaming composition is then passed into a double belt press type apparatus where the curing exotherm continues to vaporize and expand the blowing agent, thereby foaming the composition as it is cured.
As mentioned herein, one of the main drawbacks of prior art phenolic foams is an unsatisfactory initial thermal conductivity (k value). It is believed that one of the main causes of phenolic foam having a poor initial thermal conductivity is due to the rupturing of the cell walls during the foaming and early curing of the foamable phenolic resole composition. This rupturing causes an immediate loss of the blowing agent which results in a poor initial thermal conductivity. Ruptured cell walls also provide ready passage of water into the foam causing further increase in thermal conductivity. It is also believed that ruptured cell walls deleteriously affect the compressive strength and other properties of the phenolic foam. Another main cause of initial poor thermal conductivity in phenolic foams is the loss of blowing agent before the cell walls are sufficiently formed to entrap the blowing agent.
Also as mentioned herein, another drawback of the prior art phenolic foam is the undesirable increase of thermal conductivity over time (k factor drift). Even in those prior art phenolic foams which have cell walls which are not ruptured and which have blowing agent entrapped therein, the foams have a tendency to lose the blowing agent over time with a corresponding increase in thermal conductivity. It is believed that there are two main causes to an increase in thermal conductivity over time. The first is the presence of small perforations or pinholes in the cell walls including the struts that are formed where cell walls are joined together. These small perforations allow the blowing agent to diffuse out over time and be replaced by air. This slow replacement of the blowing agent with air causes an increase in thermal conductivity and loss of thermal insulation value. The small perforations also allow the phenolic foam to absorb water, thereby further increasing the thermal conductivity. It is believed that the perforations are caused by water that is present in certain parts of the foamable phenolic resole conposition, particularly the catalyst. A method for over-coming perforations in the cell walls and struts using certain anhydrous aryl sulfonic acid catalysts is the subject matter of a copending application.
The other main cause of the loss of thermal conductivity over time is cracking of the cell walls. In many of the prior art phenolic foams, the cell walls are very thin. When phenolic foam having thin walls are subject to high temperatures, the cell walls dry out and crack. Also, thermal insulation is quite often subject to heating and cooling cycles with related expansion and contraction. The expansion and contraction of the thin cell walls also causes cracking. Cracking of the thin cell walls allows the blowing agent to leak out over time with an increase in thermal conductivity and with a loss of thermal insulation value.
The art has proposed several methods for overcoming the problem of poor thermal conductivity. For example, one method involves a two-step process comprising foaming the foamable phenolic resole composition initially under a vacuum followed by curing at high temperatures and low pressures. This method does produce a foam having a substantial number of cell walls which are not ruptured; however, there are still many cell walls which are either ruptured or which are thin and readily crack when subjected to thermal stress. This method is also not commercially desirable because of the equipment that is necessary and the extended time that is required. Another method involves foaming and curing the foamable phenolic resole at low temperatures (i.e., less than 150.degree. F.). This method also reduces the number of cell walls that are ruptured but the resulting phenolic foam still has thin cell walls. Another method covered by a copending application assigned to the same assignee covers a method of foaming and curing the foamable phenolic resin composition while maintaining pressure on the foaming and curing composition. This method greatly reduces the number of ruptured cell walls but the resultant phenolic foam may still have a substantial number of ruptured cell walls or may have lost the blowing agent before the cell walls were cured, and also, the cell walls may be thin.
Other attempts at improving the thermal conductivity of phenolic foams has been based on developing specially modified phenolic resoles, or surfactants, or the use of certain additives in the foamable phenolic resole composition. None of these methods has been commercially successful. See, for example, U.S. Pat. Nos. D'Allesandro 3,389,094; Bunclark et al. 3,821,337; Moss et al. 3,968,300; Moss 3,876,620; Papa 4,033,910; Beale et al. 4,133,931; Bruning et al. 3,885,010; and Gusmer 4,303,758.
In accordance with the present invention, it has been found that the rupturing of cell walls during foaming, the loss of blowing agent before the cell walls are sufficiently formed to entrap the blowing agent, and the formation of thin cell walls is directly related to the phenolic resole used in making the phenolic foam.
Accordingly, it is an object of the present invention to provide an improved aqueous phenolic resole that yields phenolic foam having cell walls that are substantially free of ruptures.
Another object of the present invention is to provide an improved aqueous phenolic resole that yields phenolic foam that does not lose the blowing agent before the cell walls are sufficiently formed to entrap the blowing agent.
A still further object of the present invention is to provide an aqueous phenolic resole that yields phenolic foams having cell walls that are not subject to cracking because of drying or expansion and contraction.
Additional objects and advantages of the present invention will be apparent to those skilled in the art by reference to the following description and drawings.